In order to appropriately process fuel vapor generated in a fuel tank, an evaporation process mechanism (referred to as an evaporation purge system hereafter) is installed in the fuel system of a vehicle, such that fuel vapor is burned within cylinders. This evaporation purge system contains a canister in which activated carbon is sealed. After being captured by the canister, the fuel vapor generated in the fuel tank is supplied from the canister to the inlet pipe of the engine through a purge path. Furthermore, the purge path that guides fuel vapor to the inlet pipe has a purge control-valve, and by controlling the opening and closing of this purge control-valve the fuel vapor amount that is supplied to the inlet pipe can be controlled (for example, see JP Tokkai Publication No. 2001-329917 and JP Tokkai Publication No. 2001-329895).
Furthermore, in order to process fuel vapor reliably without releasing it to the outside, the evaporation purge system incorporates a diagnostic system which detects a fuel vapor leak from the fuel tank, purge path, and canister, etc. After introducing the inlet pipe's negative pressure into the evaporation purge system, such a diagnostic system determines the presence or absence of a fuel vapor leak by detecting pressure changes in the evaporation purge system over a prescribed time. Furthermore, since a large change in running condition affects failure diagnostic control, failure diagnostic control is performed when a running condition is kept constant. Therefore it is often reset when a running condition changes significantly.